Time‐resolved in situ Raman spectroscopy of working catalysts: sulfated and tungstated zirconia

Abstract

The spectra of time-dependent in situ Raman experiments on sulfated and tungstated zirconia catalysts during the isomerization reaction of n-pentane are presented and discussed. A purpose-made in situ Raman cell which was used for this application is described. The choice of appropriate experimental parameters, namely laser power and laser wavelength, is discussed. During the experiments the activity and selectivity of the catalysts were determined simultaneously by on-line gas chromatography. The sulfated zirconia catalyst shows structural changes in the sulfate region during the reaction. One of two different types of initially present sulfate species is eroded during the reaction, presumably due to a reduction to H 2 S. The activity shows a typical induction period followed by a fast deactivation. No coke formation is observed. Since fast deactivation occurs, coke formation cannot be the only reason for the deactivation of the catalyst. The tungstated catalyst shows strong darkening after initiation of the reaction with increasing time-on-stream (TOS). This leads to the disappearance of the catalyst bands. However, coke formation indicated by a broad band at 1590 cm −1 can be observed. Since darkening has a strong effect on Raman intensities, the time evolution of the observed bands is not obtained correctly. We propose a new method to correct for the effect of the darkening. The change of the diffuse reflectance of the catalyst is determined by the variation of the intensity of laser plasma lines. Based on an approximate equation proposed by Waters which correlates the Raman intensities and the diffuse reflectance of a sample, a correction factor for the spectra is obtained. After the intensity correction the spectra indicate that most of the coke is formed in the first few minutes of the reaction followed by a constant formation rate with TOS.

Active coke: Carbonaceous materials as catalysts for alkane dehydrogenation

“…Raman spectroscopy is widely used to investigate carbonaceous materials with different degrees of (dis)order [5,[55][56][57]. However, in the present study it does not yield unambiguous results.…”

Reaction pathways in n-pentane conversion catalyzed by tungstated zirconia: effects of platinum in the catalyst and hydrogen in the feed

“…Farcasiu and Lukinskas (28) detected bands at 270 and 300 nm, assigned to cyclic or acyclic substituted polyalkenyl species (28). Similar species were observed on SZ by in-situ UV-visible spectroscopy; bands at 310 and 370 nm, with a broad shoulder at 400 nm, were reported and assigned to allyl cations and polyalkenyl species (30)(31)(32). The correlation between the growth of these bands and the development of the catalytic activity in the liquid acid led the authors (28) to propose that these species were involved in the catalytic reaction (although their exact role was not established).…”

“…The postulated polyalkenyl species are expected to be precursors of the coke (42) that was detected on the WZ catalyst used for conversion of n-pentane (31). Coke deposits can increasingly block the catalyst surface with increasing TOS, leading to a decrease in the concentration of the intermediate in reaction path A, consistent with the role of coke in deactivation of the catalyst.…”

“…The H 2 is inferred to adsorb dissociatively on the platinum and to reduce the tungsten phase by hydrogen spillover. This reduction of the tungsten phase by the activated H 2 is inferred to lead to the formation of W 5+ centers and OH groups (31,33).…”

Noninvasive Glucose Sensing with Raman Spectroscopy

“…In general, these optical properties are wavelength dependent. [54][55][56][57][58] In fluorescence spectroscopy, however, diffuse reflectance correction of spectral distortions in biological media has been studied extensively. Analytical models based on photon migration theory 43 , diffusion theory 45,59,60 , as well as empirical models 61 , have been reported to obtain "intrinsic fluorescence."…”